Lambda functions in Python

Overview:

The lambda keyword in Python is used for writing lambda expressions. These lambda expressions produce anonymous functions which are also called function objects.

These anonymous functions do not have a name. The function object returned from a lambda expression is called as lambda function.

A Python lambda expression is of the following form:
lambda p1, p2…p_n:<valid Python expression>

where p1, p2...p_n are the parameters to the lambda expression.
e.g., The lambda expression, area = lambda radius: 3.14*pow(radius, 2) produces a function object when invoked with the parameter radius - area(3), returns the area of a circle as 28.26 units.

In Python, statements and annotations are not allowed in a lambda expression. Though a lambda function does not have a name, they can be assigned to a variable and the variable name can be used to invoke the function object.

Except for the restriction of not allowing any statements or annotations and not having a name, the lambda function is very similar to a regular function.

Example 1 - Lambda expression in Python:

This Python example uses a lambda function as a parameter to the built-in function filter to produce a new sequence. Based on the return value of the lambda function for each item in the original sequence, a new sequence is produced.

# Example Python program that prints only the numbers
# that are multiples of five using a lambda expression
# and the built-in function filter
multiples_5 = filter(lambda num: num % 5 == 0,
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

print("Multiples of five:")
for value in multiples_5:
print(value)

Output:

Multiples of five:
5
10

Example 2 - A Python function that returns a lambda expression:

The lambda function produce() returns a lambda function for each value it receives for the parameter base. In mathematics, a function that returns another function is called a higher order function.

# Function that returns a lamda expression
def produce(base):
return lambda exp:pow(base, exp)

# Make squares
squareFunction = produce(2)
print("Squares of numbers:")

# Two square
squareValue = squareFunction(2)
print(squareValue)

# Three square
squareValue = squareFunction(3)
print(squareValue)

# Make cubes
cubeFunction = produce(3)
print("Cubes of numbers:")

# Two cube
cubeValue = cubeFunction(2)
print(cubeValue)

# Three cube
cubeValue = cubeFunction(3)
print(cubeValue)

Output:

Squares of numbers:
4
8
Cubes of numbers:
9
27

Example 3 - A lambda expression using two parameters:

# Example Python program that finds the
# area of rectangles using lambda functions
areaRect = lambda length, width: length*width

# Area of the rectangle1 whose length is 4 and
# width is 3
area1 = areaRect(4, 3)
print("Area of the rectangle:{} units".format(area1))

# Area of the rectangle2 whose length is 6 and
# width is 5
area2 = areaRect(6, 5)
print("Area of the rectangle:{} units".format(area2))

Output:

Area of the rectangle:12
Area of the rectangle:30